As a container for storing a heated beverage, food prepared by heating, or the like, a paper-made container imparted with an insulating property is often used.
As an insulating paper-made container in the prior art, an insulating paper-made container having a double structure, constituted of a paper container main body and a paper-made tubular body fitted an outside of a barrel portion of the paper container main body, in which a space for insulation is formed between the paper container main body and the paper-made tubular body (see, for example, PTL 1), or an insulating paper-made container having an embossed paper-made insulating sheet attached to an outer periphery of a barrel portion of a paper-made main body, has been known (see, for example, PTL 2).
The insulating effect in an insulating paper-made container having a double structure constituted of a paper container main body and a paper-made tubular body is increased as the distance between the paper container main body and the paper-made tubular body is increased. In the insulating paper-made container described in PTL 1, a rib protruding to the side of the outer surface is formed on the barrel portion of the paper container main body, and the rib serves as a spacer to ensure a gap between the paper container main body and the paper-made tubular body. In order to increase the distance between the paper container main body and the paper-made tubular body, it is necessary to increase the distance of the peak and the trough of the rib provided on the barrel portion of the paper container main body by making a deep rib. However, the deep rib needs a significant extension of a paper substrate, and thus, the paper substrate may become easily breakable. Further, since the paper-made container protrudes outside significantly by the deep rib process, it tends to be difficult to separate the paper containers when pressurization of the bundles of the paper-made container occurs during transportation. If the bottom space of the paper-made container is increased in order to prevent such a problem, the stacked height is increased even when with the same number of paper-made containers, and the number of the paper-made containers able to be stored in a cardboard container is thus decreased, resulting in a problem such as a deterioration of transporting efficiency. For the same reason as above, the depth of the rib in practical use is decreased, and thus, a sufficient insulating property is not attained.
Furthermore, also in the insulating paper-made container having an embossed paper-made insulating sheet attached to an outer periphery of a barrel portion of a paper-made main body, described in PTL 2, the insulating effect is increased as the distance from the peak to the trough of the embossment is increased. In order to increase the distance from the peak to the trough of the embossment, it is necessary to make a deep embossment, and the deep embossment needs a significant extension of a paper substrate, and thus, the paper substrate may become easily breakable and it is difficult to steepen the angle connecting the peak and the trough of the embossment. Further, since the paper-made container protrudes outside significantly by the deep embossment, it tends to be difficult to separate the paper-made containers when pressurization of the bundles of the paper-made container occurs during transportation. If the bottom space of the paper-made container is increased in order to prevent such a problem, the stacked height is increased even with the same number of the paper-made containers, and the number of the paper-made containers able to be stored in a cardboard container is decreased, resulting in a problem such as a deterioration of transporting efficiency. For the same reason as above, the depth of the embossment in practical use is decreased, and thus, a sufficient insulating property is not attained, as in a case of the paper-made container described in PTL 1 above.
Furthermore, in order to enhance the insulating effect in an insulating paper-made container, it is proposed to make a double structure having a fitted paper-made tubular body so as to form a predetermined gap between an outer periphery of a barrel portion of a paper container main body and the paper-made tubular body, and the paper-made tubular body is embossed to increase the distance between the paper container main body and the paper-made tubular body (see, for example, PTL 3).
However, since the insulating paper-made container described in PTL 3 has a double structure, it has a problem in that the cost of materials as well as the number of the preparation steps are increased, resulting in an increase in preparation cost.
As an insulating paper-made container, in addition to the insulating paper-made container having the double structure as described above, an expanded insulating paper-made container consisting of a barrel member and a bottom member, in which an expanded insulating layer formed by expanding a thermoplastic resin layer is formed on the side of the outer surface of the barrel member, has been known (see, for example, PTL 4).
The expanded insulating paper-made container is a container, in which a barrel member of the container having the low-melting point thermoplastic resin layer side as an outer side is formed by using a processed paper having a low-melting point thermoplastic resin layer, which is expanded by heating, provided on one surface of a paper substrate and a high-melting point thermoplastic resin layer provided on the other side, and the low-melting point thermoplastic resin layer is expanded by heating to expand and insulate the outside of the barrel member. As compared with the insulating paper-made container having the double structure as described above, the number of the preparation steps is small, easiness of the preparation, reduction in the cost, and the like are promoted, and further, by increasing the thickness of the low-melting point thermoplastic resin layer to be expanded or by increasing the moisture amount of the paper substrate, the expanded insulating layer can be thickened and thus the insulating property can be improved.